The basis of absolute receiver calibration is the blackbody law; it is
the only radiation source that is calculable in actual practice. The
implicit assumption
is that an isothermal enclosure that does not reflect or become
transparent radiates a blackbody spectrum. There are a host of problems in
making a practical calibrator approach the idealized limit and furthermore,
given a good calibrator, in being assured that the act of placing the
calibrator at the radiometer input does not affect the performance of the
receiver or alter the geometry at the input in an incalculable way. The
calibrator should appear to the radiometer as similar to the measured
source as possible.

Most of the low frequency and all the high frequency experiments have
used cryogenic reference bodies to reduce the demands on receiver linearity
and to minimize the change in operating point of the square law detector
when switching between the calibrator and the sky. The temperature-vapor
pressure relation of liquid helium is known to better than 0.1% and with
reasonable care cryogenic thermometry can be performed reliably to this
precision. The calibrators are constructed by terminating a waveguide or
light pipe on a prismatic or conical absorber several wavelengths thick
that is in good thermal contact with the cryogen. The termination is
designed to trap the radiation through multiple reflections by the
poorly reflecting absorber material. The central difficulty in the
calibrator designs has been
the thermal gradients at the transition from the cryogenic environment to
the warns world outside, the problems being emission by the warmer
sections of the waveguide and from windows used to avoid condensation of
air in the liquid helium. These contributions are not negligible; in some
experiments, in fact, they are comparable with the emission by the
absorber. As the frequency increases this problem becomes more acute.

The emissivity of the calibrator is determined by reflection measurements,
using
= 1 - R, and
is generally larger than 99% for narrow-band
calibrators. Broad-band calibrators are more difficult to design. The
reflectivity of the calibrator has to be known because, in use, emission
by the warm receiver components (i.e. waveguides, local oscillators,
horns) is reflected by the calibrator back into the receiver and this
effect is not balanced out when the receiver looks into the sky.

Most of the detailed considerations involved with the interaction of the
calibrator, receiver, and the thermal gradients are eliminated if the
entire apparatus is maintained at cryogenic temperatures. This has been the
practice in high frequency measurements and is clearly indicated for any
new precision measurements at lower frequencies.